An electronic key system for a motor vehicle is conventionally known. In this system, in response to a request (transmission request signal) transmitted from an on-vehicle device, a portable device as an electronic key carried by a user of a vehicle transmits a reply signal containing an identification code. If the code that the on-vehicle device receives from the portable device matches a code stored in advance in a memory of the on-vehicle device, predetermined operations such as door unlocking/locking and engine startup are performed. Such an electronic key system is generally referred to as a smart key system or a smart keyless entry system [e.g., JP-2000-170420 and U.S. 2003/0095039 A1 (JP-2003-157483)].
In the above electronic key system, desired functions (door unlocking, locking, etc.) are achieved if the user only carries the portable device without performing intentional operations on the portable device. Therefore, the user often leaves the portable device in a bag, a purse, a jacket pocket or the like. As a result, the user often forgets where the portable device was put, and has difficulty in locating the lost portable device.